The present invention relates to aircraft flight control systems, and more particularly to commercial aircraft flight control systems that provide protection against inadvertent tail-to-ground contact.
Aircraft can achieve high angles of incidence relative to a runway during both takeoff and landing segments of flight. If the angle becomes large enough while the aircraft is close to the ground, the aft or tail portion of the craft may contact the runway surface. Such contact is sometimes referred to as a tailstrike and is generally sought to be avoided. For this reason and others, manufacturers recommend pitch rates and speeds at which takeoff and landing maneuvers are to be performed. In practice, however, variations in both can be expected due to differing pilot techniques and weather conditions. In some instances, takeoff and landing speeds are increased to provide additional aft body margin and thus reduce the probability of tail contact in the event of a large variation in airspeed or pitch rate. Increasing scheduled takeoff or landing speeds is not an optimal arrangement, since it introduces a performance penalty.
Others have sought to prevent tailstrikes by regulating aircraft incidence (angle of attack) by modifying the aircraft""s commanded rotation rate. See for example U.S. Pat. No. 5,527,002 in which a percentage of commanded rotation rate is used to avoid a tailstrike. In doing so, the ""002 invention does not consider several key parameters that can affect the probability of aft body contact. It is the understanding of the inventors herein that the rate at which the aft body approaches the runway is a function of both the rotation rate of the aircraft and the movement of the center of rotation relative to the runway.
Only when the majority of the weight of the aircraft is on the gear and the aircraft is rotating only about the landing gear, is the motion of the aft body toward the runway almost purely a function of rotation rate. As the wing begins to generate lift and the aircraft begins to climb away from the runway, the motion of the aft body toward the runways becomes a function of the motion of the rotation rate of the aircraft and the motion of the center of rotation relative to the runway. During this period, the center of rotation moves from the landing gear to the center of gravity of the airplane. In addition to this movement, the center of gravity of the airplane begins to move away from the runway as it lifts off. It is during this segment of the rotation, just at or just after liftoff, that many takeoff tailstrikes can occur. By ignoring the motion of the center of rotation, inventions based on pitch and pitch rate alone limit the performance of the aircraft in some situations and provide only limited protection in others.
The present invention is an improvement to an aircraft flight control system that reduces the likelihood of aircraft tailstrikes by considering such characteristics as tailskid height and tailskid rate during takeoffs and landings. The flight control system includes a pitch command provided to a pitch control device for altering the aircraft""s pitch attitude. The improvement is a system of altering the pitch command to avoid an aircraft tailstrike. The improvement includes determining a current tailskid closure rate and a current tail height; comparing the current tailskid closure rate with a threshold closure rate to determine an excess closure rate amount; and adding an incremental nose-down pitch command with the pitch command to avoid a potential aircraft tailstrike. The threshold closure rate is dependent upon the current tailskid height. The incremental nose-down pitch command is calculated as a function of the excess closure rate amount.
An alternative embodiment is provided in which the current height is compared with a threshold height (that is a function of the current tailskid closure rate) to determine an excess height amount. The excess height is then used to form an incremental nose-down pitch command. Yet another embodiment is described using both height and rate thresholds to calculate the incremental nose-down pitch command. Alternative arrangements may be used in which other non-derivative and derivative values are used, e.g., pitch angle margin and pitch rate.